Godfather1 did 135mn USD domestic and Godfather 2 releasing 2 years later did 47.5mn USD. But both were acclaimed almost equally. ( (.2 and 9.1 respectively on IMDB)
Kill Bill 1 in 2003 did 70+110-180mn USD globally and Kill Bill 2 in 2004 did 66+86 = 152mn USD . Again both were liked almost equally ( 8.2 and8.1 on IMDB)

That seems to be exactly the case with GOW 1 and 2. In fact Part 1 and Part 2 have done almost the same over their first 7 days, about 18cr. GOW 2 will earn less in the coming weeks because of ETT. But otherwise they have been liked almost equally well, GOW 1 being liked a wee bit more.

I think there was nothing wrong in the release strategy. Part 2 coming soon after the Part 1 was the best, just like in case of Kill Bill series.